When a pulsed electromagnetic radiation, such as that of a monopulse radar, is intercepted by either of a pair of antenna elements, the absolute phase and amplitude of the intercepted signal can vary in dependence upon factors which influence both the transmitter and the radiation as it passes from the transmitter to the receiver. However, the signals which the intercepted radiation produces at the two antenna elements of the pair have a relationship of phase and amplitude that is dependent only upon the angle from which the radiation arrives at the antenna elements; hence the direction to the source of the radiation can be determined on the basis of the relationship between signals at the antenna elements.
With prior receivers comprising an array of antenna elements, it has not been possible to obtain a single unambiguous value for the interception angle, but instead indications were obtained that signified numerous equally probable angle values within the response lobe of the array. Since the response lobe should be relatively broad, in order to cover as large a zone as possible, apparatus producing such ambiguous results was unsuitable for determining direction to a radiation transmitter.
In order to obtain a very precise determination of the direction from an antenna array to a transmitter that emits radiation intercepted by the array, the response lobe or sensitivity lobe of the antenna array should be relatively narrow. In efforts to improve the precision of directional determinations, various expedients have therefore been proposed for narrowing the sensitivity lobe of an antenna array. One such proposal was to employ a geometrically large antenna array. Another was to provide additional antenna elements for suppressing the side lobes of the antenna array, but this had no influence upon the breadth of the main lobe and therefore afforded little benefit with respect to attainment of the desired precision. In any case, an antenna array having a sharp and narrow zone of sensitivity is inappropriate to equipment for ascertaining direction to a radiation source for the reason, among others, that at any given instant such an array can respond only to transmissions emanating from within the small sensitivity lobe, and therefore signals from a source outside that lobe could not be received and might go undetected.
What was desired but could not heretofore be obtained in a practical form was an antenna array which had a broad response lobe for search and acquisition of signal sources to be localized, but which could be more or less instantaneously altered or modified to have a sharp, narrow sensitivity lobe for precise localization of a radiation source once its general direction had been ascertained. Of course such quick alteration of the construction of an antenna array is not a realistic possibility; but even if it could be achieved, it would still leave unsolved the problem of ambiguities in the signals obtained from it.